1. Field of the Invention
This invention relates to color printing and more particularly to the alignment of the printing rolls, or plate cylinders, in a multicolor printing press for circumferential registry. More particularly still the invention relates to a method and apparatus by which the printing roll of a multicolor printing press such as, for example, so-called flexographic, gravure and lithographic presses can be conveniently and economically prepared for reuse in the press without trial and error type realignment of the rolls for circumferential registry.
2. Description of the Prior Art
Rotating roll type printing presses are currently used for printing multicolor images or impressions as well as single color orders printed at multiple stations on flexible materials such as paper, cloth and plastic materials. Such rotary type printing presses, may, for example, be so-called flexographic, gravure and lithographic presses in which the print repeats vary. In such presses a flexible printing plate is attached to a rotating print roll or plate cylinder. A series of rolls or plate cylinders are used, one for each color which is to appear in the image or in adjacent images usually referred to as "impressions". For example, if there are to be four colors in an impression there must be four active printing stations in the press. Each station is provided with a printing roll, or plate cylinder, upon which is mounted, usually by glue adhesion, a flexible rubber or the like plate which carries the portion of the printed image or impression which is to be one of the four colors. Next to the plate cylinder is mounted an impression cylinder or roll over or around which the material to be printed passes between the two cylinders. An ink transfer cylinder is usually mounted on the opposite side of the plate cylinder. The ink transfer cylinder in turn is usually mounted against an ink cylinder which operates partially submerged in an ink well or reservoir. In operation, the ink cylinder as it rotates draws ink from the ink reservoir on its surface by wetting the surface and transfers such ink by contact to the surface of the ink transfer cylinder which in turn every time the plate cylinder rotates transfers ink of the color drawn from the ink reservoir to the flexible printing plate on the plate cylinder by contact. This ink is then transferred as a part of an image or impression from the printing plate to the material being printed as the plate cylinder is rotated about its axis to bring the printing plate into contact with the material as it passes over the impression cylinder. The impression cylinder serves to support the flexible material upon which the printed impressions are being formed. The material being printed is usually in the form of a strip or sheet of some sort which is pressed against the printing plate on the plate cylinder as the material rotates while supported on the surface of the impression cylinder past the plate cylinder. Each of the four colors of which the complete image is to be comprised is applied to the material being printed from a plate cylinder and printing plate at a different station in the printing press. Since all the colors cannot be applied simultaneously, but must nevertheless be applied to the same general portion of the printed material, it is necessary that the colors be applied consecutively in precise coordination with each other to form a complete image.
One of the major problems in multicolor printing is coordination of the impressions with each other in so-called registry. Since the various colors must be applied serially or consecutively, it is important, and in fact critical, that they be applied in exactly the right place on the printed material. If not, the image will be distorted and if the colors overlap even a fraction of an inch, the image will be unsatisfactory. Thus the registry of the printing plates as they consecutively print portions of the image or impression is of great importance. Registry is a problem with flat bed presses, but is an even greater problem with conventional rotary type presses and a greater still problem with so-called flexographic presses which because of their speed use very fast drying ink.
Transverse or lateral registry or misregistry is not as severe a problem as longitudinal misregistry. Longitudinal registry is frequently referred to as circumferential registry or misregistry, i.e. misregistry measured with respect to the circumference of the plate cylinder. Various guides can be used to hold the plates and plate cylinders in transverse registry, but since the material moves longitudinally, no guides can be used to effect longitudinal registry of the material. Each printing roll, or plate cylinder, has a fixed circumference, furthermore, and any portion of the printed image or impression will thus repeat regularly over a distance along the printed material equal to such circumference. But when two rolls or plate cylinders are placed adjacent to each other in a printing press the distance printed material travels in passing from tangent contact with one to tangent contact with the next is seldom if ever the same as the repeat distance of either roll or plate cylinder. Consequently, if an adjacent plate cylinder is to essentially repeat the image or impression of a prior plate cylinder, as is necessary if the second plate cylinder is to fill in a different color in the same image or impression printed by the first, the surface of the second plate cylinder must be either retarded or advanced so the repeat distances will be coordinated and the impressions aligned. The problem is accentuated if a repeat pattern of the same image is to be repetitively made, as is usually the case in any high volume operation, and as a result the same diameter plate rolls are customarily always used together in a printing press as a matched set. Thus, there will be a stock of a number of sets of plate rolls for a press, each set being of the same diameter or, more importantly, the same circumference.
The printing plates, which are conventionally about one eighth inch thick, are adhesively secured to the surface of the plate cylinders. If, for example, a four color or six color press is being used there could be either four or six or as few as two of the same diameter or circumference plate rolls for each set. The particular diameter set which is used is determined by the necessary repeat distance. However, since it would be impractical to provide an infinite number of sets of plate rolls or cylinders to provide all possible necessary repeat distances, the size of the cylinder is used only to provide roughly the registry required and other means are used to provide the final adjustment.
The first of such other means is a trial and error alignment of the rolls or plate cylinders to effect so-called rough registration when the plate cylinders are first mounted in the printing press. The set of matched cylinders, after having the appropriate printing plates adhered to their surfaces, are mounted in the printing press in approximate or estimated registry. The press is then operated for a short run and the printed materials are examined to see how much misregistry is present. The cylinders are loosened and turned by hand to remove such misregistry and the press is again run for a short period. Four or five trials by an experienced pressman are usually sufficient to roughly register the plate cylinders in the press. Fine or final registry is then usually accomplished by special adjustment of the press usually referred to as machine adjustment.
Final registry adjustment may be effected depending upon the printing press mechanics either prior to engaging the press drive or during operation of the press to effect perfect or final registry. A number of mechanical devices have been designed to effect the fine registry, usually prior to placing the press in operation. Usually such devices involve the use of so-called change gears by which the plate cylinders may be individually rotated prior to engaging the press drive gears. The individual rotated adjustment is then retained when the press drive is engaged. Other dynamic devices are available which enable the rolls to be individually rotated even while the press drive is engaged so adjustment can be made during actual printing. All these devices are referred to as machine registry devices. Since these devices are designed for fine registry in fractions of an inch, however, and usually in the neighborhood of one half inch, they are not suitable for removing gross misregistry of the plate cylinders. Consequently, rough registration of the cylinders is usually effected manually. The rough registry, if done entirely by trial and error, will often take an experienced pressman from one to one and a half hours and since such rough registration must be effected every time the plate cylinders or printing plates are changed for a new job, such rough registration can result in a very significant amount of down or unproductive time. Modern presses are quite expensive and must operate fairly continuously to recover their cost. Thus down time for rough registering the plate cylinders of the press can be a very important consideration.
Since the registration or misregistration of the rolls or cylinders of the press is dependent upon the distance each cylinder must turn in order to secure a substantial repetition of the impression previously left by the printing plate on a prior roll, but in a different color, and this repeat distance is dependent upon the circumference of the cylinder and the distance between cylinders, proper registration of the plate cylinders can be calculated. However, accurate calculation not only is time consuming, but requires very accurate measurements and it has been customary, therefore, to obtain rough registration by trial and error rather than calculation. The difficulty in attaining rough registration of the plate cylinders is increased by the fact not only that different sized sets of plate cylinders are used in printing presses, but the distances between cylinders or printing stations are adjustable in some modern printing machinery.
Devices and procedures for effecting the rough registration of rolls or plate cylinders in a roller type press have been designed of which the following patents are illustrative.
U.S. Pat. No. 3,160,094 issued Dec. 8, 1964 to L. H. Bean discloses a rough registration system in which the plate cylinders are first rough registered by trial and error in accordance with the traditional method. A longitudinal so-called prime line is then established on the plate cylinder. This marks the location at which the printing plate cylinder is mounted each time. A reference line is then established on the end of each plate cylinder or on the end of the arbor of the plate cylinder at a fixed position relative to the prime line on the surface of the cylinder by which the printing plate is aligned. Preferably the reference line will be at right angles to the prime line. The angle of the reference line with respect to a gravitationally established constant horizontal or vertical reference is then measured for each cylinder of the rough registered set-up and the angles are recorded. When the same set of plate cylinders are then again used, the recorded angles may be retrieved from the records and the cylinders aligned so that the originally recorded angles are duplicated without trial and error alignment. It is claimed that the Bean method allows a set of plate cylinders to be rough registered within a few minutes. Whatever the time, which is less than registration by trial and error, it remains necessary to remeasure the angles, usually by a leveling device, which in itself is time consuming, and the Bean method has consequently not come into widespread use.
U.S. Pat. No. 3,662,472 issued May 16, 1962 to J. R. Johnson discloses a template for use in setting the repeat distances of adjacent plate rolls by angular position. The template is provided at one end with a series of ring segments which fit over the mandrels of a series of printing plate rolls in succession. The ring segments are rotated to indicate the angular displacement of the roll necessary to bring the repeat position of such roll indicated by the pitch mark on its surface into coordination with the distance of such roll along the material being printed from the first printing plate roll of the press. A pointer is used to coordinate the pitch mark on the roll surface with the angular indications on the ring segments. The apparatus of the invention of the patent requires the moving of the ring segments or template from roll to roll during set up and the calculation of the mathematic relationship between the angular repeat distance on the roll and the repeat distance along the printed material.
There has been a need therefore for a simple and reliable means and method for effecting a rough registration of the plate rolls or cylinders in a roller printing press without time consuming manipulations, measurement, or trial and error set up runs.
It is an object of the present invention, therefore, to provide a method and apparatus for quickly rough registering printing plate rolls or cylinders once an initial printing set up is effected by trial and error.
It is a further object of the invention to provide printing plate rolls or cylinders marked in a manner which allows them to be set up for reprinting without any substantial calculation or manipulation.
It is a still further object of the invention to provide a measuring means in the form of a permanent strip which can be used to prepare new or old plate rolls for quick rough registration in a roller printing press.
Still another object of the invention is to provide a method for facilitating and effecting rough registration of printing plate rolls or cylinders which may be carried out by relatively unskilled personnel.
Another object of the invention is to provide a method and apparatus for registration of printing plate rolls or cylinders which is applicable to both stack and drum type printing presses and which is equally applicable to both integral and demountable sleeve printing plate rollers or cylinders.
Still another object of the invention is to provide a method and apparatus for effecting rough registration of plate cylinders which requires no costly modifications in the plate cylinders or roller presses as presently used.
Further objects and advantages of the invention will become apparent from the following description and appended claims and drawings.